On the basis of their rapid, clean, and simple processability, in the form of diecuts, for example, their permanent tack, and also the fact that there is no need for a curing step after application, pressure sensitive adhesive tapes are nowadays needed in diverse sectors and for enumerable applications, adhering reliably to different substrates and over a defined time period. Customary specialty products employed in industry additionally have a number of further functions to take on in addition to the adhesive bonding. For use in the electronics industry, for instance, in the region of optical bonds, for example, the requirement is for halogen-free, colorless, aging-stable pressure sensitive adhesive tapes. In the construction industry likewise, as for example in the bonding of architectural facings—and particularly of glass facings, where there are great fluctuations in temperature and light irradiation—, there are large areas of use for colorless, transparent bonds stable to weathering and to aging and stable in color, such bonds often being sited in view.
Such applications with high transparency are currently covered preferably by polyacrylate-based, hydrogenated styrene block copolymer-based or silicone-based adhesive tapes.
Over the years, the production of high-quality displays for smartphones, tablet PCs, and other electronic devices has ramped up considerably on the basis of heightened demand and optimized production conditions. Equally, the areas of application of the devices employed have seen continual growth, and such devices are more and more often used under extreme conditions. As a direct consequence of this, the requirements for inexpensive, high-quality, precisely tailored or universal bonding solutions have likewise risen.
Arising from the use of the adhesive tape for joining different materials adhesively is a demand made on the adhesive in terms of its adhesive and cohesive properties. Among the adhesive properties (surface property), the peel adhesion (resistance to peeling) is a central characteristic of adhesives and adhesive films in terms of their interaction with a relevant substrate. Another main characterizing variable are the cohesive properties, which describe the internal strength of the adhesive in the face of physical stresses, such as the behavior toward static shearing stress, for example. Because the two properties are in mutual opposition, the aim is always to find a well-balanced optimum tailored to the adherends, where it is not possible to improve both—adhesion and cohesion—at one and the same time.
Such adaptation of the adhesive materials to the service conditions (e.g., temperature, mechanical stress, or recycling) is in general complicated and difficult. Oftentimes the adhesive is optimized through appropriate combination of high molecular mass base elastomers with a very low glass transition temperature (Tg), e.g., natural rubber, synthetic rubbers, acrylate rubbers or elastic polyurethanes, with low molecular mass tackifier resins having a high Tg, based for example on rosins, terpenes or hydrocarbons. In exceptional cases it is also possible to achieve a similar outcome by modifying a high-Tg elastomer with plasticizers. Significant exclusion criteria for the selection of suitable raw materials are, additionally, the compatibility with one another and also the adaptation to the envisaged bond substrates, but with severe restriction on the raw materials appropriate.
For pressure sensitive adhesives (PSAs), the (meth)acrylate-based rubbers with high molecular weights (e.g. Mw>500 000 g/mol) represent an important class of raw material having very good properties. Such adhesives are notable for high temperature, chemical, and aging stability and are suitable, accordingly, particularly well for high-quality bonds (e.g. in the electronics sector). Preparation is accomplished customarily by copolymerization of suitable monomer mixtures in organic solvents, in aqueous emulsion or in bulk. The primary influencing variable for controlling the technical adhesive properties on the basis of the base polymer properties is above all the molecular weight of the base polymers thus produced and also the resultant glass transition temperature (Tg), which can be estimated and tailored on the basis of a suitable selection of the monomers. Suitable in general here are glass transition temperatures in the range below 0° C., but primarily between −95° C. and −30° C. for adhesives particularly.
Via the nature and the degree of crosslinking it is possible to form a network between the linear polymer chains and to further control the cohesion of the polymer matrix by using increasing crosslinking to modify the viscoelastic behavior from viscose gradually up to elastic. Through the suitable addition of crosslinkers or through the influence of radiation and/or heat, especially suitable adhesives are obtained that exhibit low degrees of crosslinking. When using radiative crosslinking, especially UV crosslinking, moreover, it must be ensured that the additives present in the adhesive formula (resins, for example) do not interact with the crosslinking reaction, hence being UV-stable and UV-transparent within the relevant range, in order to prevent unwanted side-reactions (e.g., discoloration) and uneven crosslinking.
For the purposes of formulation, the amount and nature of the additives, but particularly of tackifier resins or plasticizers, are increasingly decisive. For both raw materials, tackifier resins and plasticizers, the compounds in question are of low molecular mass, with molar masses<50 000 g/mol, but primarily<2 000 g/mol. In accordance with their function, they differ in their physical properties such that plasticizers are typically low-melting compounds with a softening point below 40° C., but more particularly are liquids, such as many oils or liquid phthalic esters, for example, which on account of their low intrinsic glass transition temperature (Tg<0° C.) reduce the overall glass transition temperature of the mixture—starting from the original high-Tg polymer such as polyvinyl chloride (PVC) or polyethylene terephthalate (PET), for example. Tackifier resins, on the other hand, especially for acrylate adhesives, have high softening points above 70° C., but typically between 90° C. and 160° C. Important representatives are terpene-, phenol-, rosin- or naphtha-based resins. When used in base polymers, their high intrinsic glass transition temperatures (Tg>30° C.) serve to raise Tg and to boost the shear strength under temperature load. Concentrations used here are between 0% and 70%, but primarily between 10% and 40%.
In formulation development, there are always many additional requirements to be met. For instance, in addition to quality and economic viability, good processing qualities in particular (especially coating, slitting, and diecutting characteristics) are an extremely important characteristic which must be ensured. Although the task identified above is already extremely complex, there is often the need, furthermore, to adapt the adhesive to specialist requirements.
In connection with their wide usage, therefore, specific single-sided and double-sided pressure sensitive adhesive tapes have increasingly also come into the focus of optical and electronic applications. The key design forms in this context comprise double-sided, single-layer, carrierless adhesive transfer strips which are lined with one or two antiadhesively coated outer plies (release liners); double-sided, two-layer or multilayer adhesive strips having a single-layer or multilayer carrier and lined with one of two antiadhesively coated outer plies; and also single-sidedly pressure sensitively adhesive strips having a single-layer or multilayer carrier, and lined with an antiadhesively coated release liner.
Especially for part-area and full-area bonds in display applications, touch sensor applications, coverglass bonds or surface protection bonds, essential importance attaches to adhesive solutions featuring high transparency and colorfastness of the bond. Within the patent literature there exists a multiplicity of applications claiming pressure sensitive materials for use in optical bonds. These are essentially polyolefin-based and polyacrylate-based pressure sensitive adhesive tapes. The pressure sensitive adhesives are preferably aqueous or solventborne polyacrylate formulations. Pressure sensitive adhesive tapes of this kind are shown in specifications WO 2005063906 A and DE 203 15 592 U. In the recent period, furthermore, technical adhesive tapes have been described for use in the electronics sector, based on solvent-free, UV-crosslinkable polyacrylates; in this regard, see EP 1548 080 A.
The transparency, as an optical property of the material, is critically influenced by the transmittance τλ or the absorbance Aλ in accordance with the Beer-Lambert law. This describes the attenuation of the intensity I0 of the light of a defined wavelength λ penetrating the material to the value of I. The absorbance is dependent on the path length d of the material, on the decadic molar attenuation coefficient ελ of the absorbing substance, and on its concentration c in the sample material. Aλ is given by the equation (I), which is likewise valid for the UV range and can therefore be employed for assessing an adhesive component for its suitability when deployed in UV crosslinking:Aλ=Ig(1/τλ)=Ig(I0/I)=ελcd  (I)
Adhesive bonds are referred to as transparent (in the sense of the invention presented in the present specification) for a transmittance τλ>95%, preferably τλ>99%. A key role is also played here by the coloredness of a substance with regard to the transparency, since the coloredness represents a wavelength-dependent modification, occurring in the visible range, of the attenuation coefficient ελ. In simplified form, the coloristic value of a sample is indicated in the form of coordinates in the three-dimensional L*a*b* color space, which covers the range of colors perceptible to the human eye. Here, L* describes the coordinate axis for lightness, a* that for green-red, and b* that for yellow-blue. For resins, positive b* values are particularly frequent, since virtually all resins have a certain yellow coloration. This is manifested particularly in so-called high-build applications (e.g., joint assembly between the edges of two glass plates) in view of the high path length d.
Likewise always visible for example are defects in the bond caused by inclusions of air or of impurities, because of light scattering (haze). This light scattering may come about as a result of physical processes (e.g., surface unevenness/scratches) or chemical processes (e.g., uneven distribution/migration of formula components within the adhesive, or uptake of moisture). One measure of the scattering of a layer or of any transparent object under measurement is its haze, also called large-angle scattering. This, according to ASTM D 1003, is the fraction of the total light passing through an object under measurement that within the object under measurement undergoes a directional deflection of more than 2.5°, in other words being scattered out of the directed beam. Haze values of H>5%, in the narrower sense even H>3%, are ultimately unusable for optical bonds and are not tolerated in the adhesive film/display of the customer, in the same way as discolorations (e.g. |b*|>1) or hazing (caused by aging-related moisture uptake, for example).
It is these particular requirements asked of visible bonds, particularly in transparent, optical applications, where a great number of the classes of raw materials used preferentially in adhesive tapes suffer failure. In particular, all tackifier resins based on rosins, polyterpenes or hydrocarbons exhibit strong intrinsic coloration or a temporary change in color or transparency under the action of light, temperature, humidity or oxygen. A factor which causes no problems in other adhesive tape applications, when, for example, the bond site is placed out of sight, is a significant exclusion criterion for transparent applications (see EP 157 455 A).
In spite of their particular tack-enhancing effect, therefore, hardly any tackifier resins are employed in specialist applications such as display bonds, and the greatly restricted possibilities of resin-free systems have to be accepted. Occasionally, reducing the layer thickness of the adhesive strip, so producing a boost to the transparency in accordance with the Beer-Lambert law, offers a compromise. The other side of the coin, however, is a loss of technical adhesive performance and latitude for compensating uneven or specially shaped substrates with different layer thicknesses, or adequately withstanding pressure and jolting loads which the adhesive volume is required to accommodate.
Another exception is formed by compositions for applications with less stringent transparency requirements, comprising one of the few remaining resins which is colorless or has low levels of coloration, such as hydrogenated rosin derivatives, for example. In many cases, however, even these refined resins display a tendency to change color due to aging, and within the adhesive must be stabilized by further additives such as primary and secondary aging inhibitors and also UV absorbers, and/or the effect must be compensated by pigmentation. Examples of aging inhibitors contemplated include sterically hindered phenols and amines, phosphites and organic sulfides. Preference is given to using sterically hindered phenols, such as those of the Irganox® tradename and aging inhibitors where primary and secondary aging inhibition effects are united in one molecule.
As a result of using aging inhibitors, it is usual for new, unwanted side effects to be generated elsewhere (e.g., detractions from the technical adhesive performance profile, no possibility of UV crosslinking). Moreover, the addition of aging inhibitors does not offer permanent protection, but only delays the unwanted effect for a certain time. Storage or processing under hot and humid conditions (e.g., in the case of production in Asia), or service under critical conditions (e.g., in sauna interiors, close to hot machinery, or in outdoor applications with high UV load) is then possible only for a certain time.
For hydrogenated resins to be used, however, they must be prepared by involved methods (e.g., catalytic hydrogenation under high pressure of up to 200 bar and at high temperatures) and, because of their low polarity, they are not compatible with all types of polymer. It is specifically, however, the compatibility between resin and base polymer that is a further key limiting factor in the selection of resins. Even slight incompatibilities (inadequately aligned solubility properties) of resin and polymer can lead to the development of hazing because of phase separation (primarily separation of the high molecular mass resin constituents). If the molecular structures of polymer and resin are known, the solubility properties of the compounds can be computed on the basis of tabulated values for functional groups. Good agreement between the values is an indicator of good compatibility. In the case of poor compatibility, suitable chemical modification to the molecular structure can improve the compatibility.
A rough impression of the solubility properties is supplied by the cloud point measurement methods of MMAP (of resins in a 2:1 mixture (vol %) of methylcyclohexane and aniline, corresponding to the in-house method H90-5 of Hercules) and DACP (of resins in a 1:1 mixture (wt %) of xylene and 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol) in accordance with the in-house method H90-4a of Hercules). In each of these cases a determination is made of the temperature at which the resin begins to become insoluble. Low values for MMAP (<40° C.) describe highly aromatic resins, whereas high MMAP values (>75° C.) indicate aliphatic or hydrogenated test substances. And low DACP values (<0° C.) mark out high-polarity resins with high specific adhesion to polar substrates such as polyester, aluminum or steel, whereas high DACP values (>0° C.) mark out weakly polar resins for low-polarity substrates.
None of the specifications cited, however, discloses an approach which as well as the establishment of the specific technical adhesive and performance profile also exhibits high transparency, aging resistance, and color stability, and ensures improved coating behavior in relation to processability, quality, and economic viability. A common report is that the resin component adversely affects the color and the aging stability of the PSA.
There is therefore a need, particularly, but not limited, to application in the electro-optical sector—to provide PSAs which exhibit an excellent balance between cohesive and adhesive properties and at the same time exhibit enhanced transparency, enhanced aging resistance, and enhanced color stability. Moreover, in view of the worldwide scarcity of resources, there is an additional problem of switching to a greater extent to biobased or recycled raw materials. In addition, physical properties such as degree of purity (color), odor, glass transition temperature, economic viability, compatibility with other formulation constituents, and reduced hazard potential, significantly different from the properties known from the prior art, are desirable, solving the disadvantages associated with them.
In view of the additional optical requirements (high transparency, low coloredness) to the mechanical requirements (technical adhesive performance profile such as adhesion and cohesion, for example) asked of adhesive tapes for visible and/or optical bonds, but preferably in the optical, optoelectronics or architectural facing construction sector, as for example for display panel bonds, touch sensors/panels, cover glass bonds or surface protection strips, however, the use of colorless tackifier resins would very frequently be desirable. Factors in need of improvement here are primarily those of haze, color or color stability of PSAs modified with tackifier resin, and of adhesive tapes containing tackifier resin, and more particularly the intrinsic coloring, the color stability and the compatibility of tackifier resins with the various adhesive components used.
It has been possible in accordance with the invention to achieve the object by means of a pressure sensitive adhesive as shown in more detail in the main claim. Dependent claims referring back to this claim relate to advantageous embodiments of the pressure sensitive adhesive. A further subject of the invention is the use of the pressure sensitive adhesive for bonds of substrates and/or in devices for optical and electronic applications, or in the construction industry, and also advantageous developments of this use.